The epidemic of human immunodeficiency viruses and acquired immunodeficiency syndrome (HIV/AIDS) presents opportunities for understanding the etiology of and ultimately preventing cancer. Prospective cohort studies of persons with hemophilia, homosexual men, pregnant women and infants, and persons in Africa and the Caribbean provide complementary insights. Slower progression to AIDS was found for the nearly 20% of HIV-infected whites who had a 35-base pair deletion in one of their two alleles of the CKR5 gene, a recently discovered co-receptor for HIV-1; the 1% of whites with this deletion in both alleles appeared to be resistant to HIV-1 infection. Natural history studies on time-to-AIDS also provided the basis for "back-calculation" modeling approaches to quantifying the HIV epidemic in the entire U.S. Approximately 700,000 Americans are living with AIDS; HIV incidence in the 1990s has been highest in young adults and underprivileged minorities. In the Multicenter Hemophilia Cohort Study, HIV was quantified during early chronic infection, that is, 1 to 3 years after the subjects had been infected through the use of plasma products that had been used to treat their hemophilia. The level of HIV among different subjects ranged from 200 to more than 1 million HIV-1 RNA copies per milliliter of serum, and the level was closely tied to the rate of progression to AIDS for more than 10 years. In Malawi, an intervention trial of nearly 7000 women found that cleansing the birth canal with chlorhexidine, an antiseptic, greatly reduced bacterial infections and related mortality in mothers and infants, but it did not reduce perinatal HIV transmission. With an ever-increasing number of persons who have survived with HIV-1 infection for 10 years or more, there is increasing likelihood of observing and understanding diseases of long latency, such as cancer. VEB strives for such understanding with multidisciplinary investigations of HIV-1, other viruses, environmental factors, and host responses in populations that are likely to be informative. The goal is to prevent or ameliorate HIV-1 infection and related diseases as soon as possible through science and public health intervention.